1. Field of the Invention
The present invention relates to a mobile communication system supporting asymmetric Carrier Aggregation (CA). More particularly, the present invention relates to a method for discriminating messages received in response to random access preambles transmitted in different uplink channels.
2. Description of the Related Art
Mobile communication technologies have been developed to provide communication services on the move. With the rapid advance of the mobile communication technologies, a high speed data communication service is becoming a basic communication service in addition to a voice communication service.
The 3rd Generation Partnership Project (3GPP), which is responsible for the standardization of Universal Mobile Telecommunications System (UMTS), is working to significantly expand the performance of UMTS by developing a Long Term Evolution (LTE) system. LTE is a 3GPP standard that is being developed to allow a downlink speed of up to 100 Mbps, and its commercial deployment is expected by 2010. In order to fulfill the requirements for the LTE systems, studies have been done in various aspects, e.g., reduction of the number of involved nodes in connections and placing radio protocols as close as possible to radio channels.
As the work on the LTE approaches its completion, activities on a further evolution of LTE are beginning to take shape within 3GPP. With the initiation of LTE-Advanced (LTE-A), several technology components are being discussed. Among them, Carrier Aggregation (CA) is a technology to increase bandwidth by aggregating multiple carriers. The CA technology can be divided into two categories according to the number of uplink and downlink channels allocated to each user: symmetric and asymmetric CAs. In the symmetric CA, the number of allocated uplink channels is the same as the number of allocated downlink channels. For instance, an uplink-downlink allocated channel ratio of 2:2, 5:5 or any symmetric ratio of x:x can be adopted for a symmetric CA. On the other hand, asymmetric CA is implemented using an uplink-downlink allocated channel ratio such as 1:2, 3:1 or any asymmetric ratio x:y.
FIG. 1 is a diagram illustrating an asymmetric CA system having an allocated uplink-downlink channel ratio of 2:1 according to a related art.
Referring to FIG. 1, a base station uses one downlink channel 105 for transmission to a User Equipment (UE), whereas the UE uses two uplink channels 110 and 115 for transmission to the base station.
In order to support CA, the UE is required to perform several operations. One of them is a random access procedure. A random access procedure is typically performed when a UE awakes from a sleep mode, performs a handover from one cell to another or when it loses uplink timing synchronization. More specifically, the random access procedure allows the UE to acquire uplink timing synchronization which allows it to send uplink scheduling or a resource request.
FIG. 2 is a sequence diagram illustrating a random access procedure between an Enhanced Node B (ENB) and a UE in an LTE system according to a related art.
Referring to FIG. 2, a UE 205 desires service within a cell controlled by an ENB 210. As part of the service acquisition, the UE 205 accesses the cell to perform a random access procedure via a Random Access CHannel (RACH). In step 215, the UE 205 receives system information, related to the random access procedure, broadcasted within the cell by the ENB 210. The system information from the ENB 210 includes groups of random access preambles and sets of available access preambles in each group (i.e., a range of preamble IDentifiers (IDs) belonging to an RACH preamble group A, a range of preamble IDs belonging to an RACH preamble group B), a threshold value (THRES) of the size of a message to be transmitted by the UE 205, and offset information (OFFSET) indicating channel status. It is noted that these parameters are specified in 3GPP Technical Specification 36.331 V8.5.0, the entire disclosure of which is hereby incorporated by reference.
If a preamble group and a preamble are selected, the UE 205 transmits a Random Access Preamble (RAP) indicating the selected preamble group and preamble to the ENB 210 in step 220. Upon receipt of the RAP, the ENB 210 transmits a Random Access Response (RAR) message to the UE 205 in step 225. The RAR message contains a Random Access Preamble ID (RAPID), Timing Adjustment (TA) for compensating uplink timing, uplink resource allocation information (UL grant) for scheduled uplink transmission of the UE 205, and a Temporary Cell Radio Network Temporary Identifier (Temporary C-RNTI). The RAR message can carry such information for multiple RAPs. That is, the RAR message can contain multiple RAPIDs, TAs, UL grants, and Temporary C-RNTIs.
The RAR message is transmitted on a Physical Downlink Shared CHannel (PDSCH) and the location of the RAR message in the PDSCH is transmitted on a Physical Downlink Control Channel (PDCCH). The PDCCH includes a payload and a 16-bit Cyclic Redundancy Check (CRC) scrambled with a Radio Network Temporary Identifier (RNTI). The RNTI used in the random access procedure is called a Random Access-RNTI (RA-RNTI). The RA-RNTI is determined based on the time-frequency resource allocation of the RACH used for transmitting the RAP. The RA-RNTI is used on the PDCCH when the RAR messages are transmitted. It unambiguously identifies which time-frequency resource was utilized by the UE to transmit the RAP and thus is used by the UE to decode the PDCCH message containing its assigned information from the ENB. More specifically, the RA-RNTI is determined according to Equation (1):RA-RNTI=1+t—id+10*f—id  (1)where t_id denotes a time index of a resource element in a Physical Random Access Channel (PRACH) in the range of 0≦t_id<10, and f_id denotes a frequency index of a resource element in the PRACH in the range of 0≦f_id<6. The PRACH is a physical channel for carrying the RAP.
To acquire the resource allocation information from the RAR message, the UE 205 performs blind decoding on received PDCCHs using the RA-RNTI. That is, to decode the payload of the PDCCH assigned to the UE 205 by the ENB 210, the UE 205 uses its associated RA-RNTI on all PDCCHs. Accordingly, if a specific PDCCH can be decoded with the RA-RNTI associated with the UE 205, then the PDCCH is destined for the UE 205. If the UE 205 successfully decodes a specific PDCCH using the RA-RNTI associated with the RACH resource used for transmitting its RAP, the UE 205 is able to locate the radio resource information contained in the RAR message on the basis of the decoded PDCCH information. The UE 205 identifies resource information assigned by the ENB 210 by detecting an associated RAPID. Using the allocated resource information from the RAR message, the UE 205 can perform an uplink scheduling transmission or resource request transmission in step 230.
If multiple UEs have transmitted the same RAPs at step 220, the RAPs collide with each other. In this case, the ENB 210 transmits to the UE 205 a Contention Resolution message in step 235, informing of which UE's transmission is successfully received. The Contention Resolution message includes a System Architecture Evolution (SAE) Temporary Mobile Station Identifier (S-TMSI) received at step 230 or a random number. Each UE that received the Contention Resolution message determines whether the S-TMSI or the random number contained in the Contention Resolution message is identical with the one it transmitted at step 230. If the received S-TMSI or random number is identical with the transmitted S-TMSI or random number, the corresponding UE continues the random access procedure and, otherwise, restarts the random procedure.
In an asymmetric CA system as illustrated in FIG. 1, a problem may arise when performing a random access procedure. Specifically, the UE may transmit RAPs through both the uplink channel 110 and the uplink channel 115. Since the ENB transmits the RAR messages through only the one downlink channel 105 in response to the two RAPs, the UE must discriminate which RAR message is responsive to which of the two RAPs. However, according to the prior art, the UE would have the same RA-RNTI for both RAPs. Therefore, a need exists for a system and method that allows a UE to discriminate RAR messages responsive to RAPs transmitted in the different uplink channels.